1. Field of the Invention
The present invention relates to a nitrogen oxides (NOx) decreasing apparatus for an internal combustion engine. More particularly, the present invention relates to an apparatus for effectively purifying NOx included in exhaust gas from a lean burn engine, using a combination of an NOx absorption and release material and an NOx purification catalyst.
2. Description of the Prior Art
To improve fuel economy and to suppress exhaust of carbon dioxide (CO.sub.2) to thereby reduce global warming, engines capable of fuel combustion at lean air-fuel ratios (lean burn engines which include lean burn gasoline engines and diesel engines) are being developed and are partially in actual use. Since a conventional catalyst (three-way catalyst) cannot reduce and purify NOx at lean air-fuel ratios, there is a need to develop a catalyst or system that can purify NOx even at lean air-fuel ratios, that is, under oxidizing gas conditions (excess-oxygen conditions).
As an apparatus capable of purifying NOx under oxidizing gas conditions, the present applicant filed Japanese Patent Application HEI 3-289046 an Oct. 4, 1991, in which an NOx absorption and release material capable of absorbing NOx at lean air-fuel ratios and releasing the absorbed NOx at rich air-fuel ratios is installed in an exhaust conduit of a lean burn engine. The air-fuel ratio of the engine is momentarily changed to a rich air-fuel ratio before the NOx absorption and release material is saturated so that the NOx absorption and release material is regenerated.
However, NOx release operation of the NOx absorption and release material is affected not only by the amount of excess oxygen in the exhaust gas but also by exhaust gas temperature. Therefore, the NOx absorption and release material may release the absorbed NOx at some temperatures even under oxidizing gas conditions. On the other hand, the NOx absorption and release material may release little NOx at other temperatures even at rich air-fuel ratios, and therefore, the NOx release time period may have to be prolonged or the frequency of NOx release may have to be increased, which will be accompanied by degradation of driveability. Further, if it is intended to maintain the NOx absorption and release material at appropriate temperatures from the viewpoint of NOx absorption and release, location of the NOx absorption and release material will be limited and freedom of design will be restricted.